Trying to make it cut-and-dry is pointless. By the nineties, thrash, death metal, and black metal were distinct genres, but throughout the eighties the distinctions between them (not to mention grindcore and even hardcore) could be so vague that classifying them is only useful in hindsight. Sarcofago was somewhere inbetween all three, early Sepultura likewise. Even early Slayer could just as easily be called first-wave black metal. Other bands that are called thrash, such as Sodom and Destruction, have early albums that are considered black metal. Obituary has a heavy Celtic Frost/Hellhamer influence the same as the Norwegian black metal bands. Listen to early Death demos and you can see a straight line going from those same influences until they finally record their debut which is death metal that's still not removed from thrash.

When genres are developing they're just too linked to multiple other genres for classification to be black and white.

Powerviolence, especially a band like Man is the Bastard, doesn't sound like "punk with light metal elements" at all, it sounds like something quite far removed from punk.

And what Industrial elements are you referring to in Grindcore? There's nothing about Grindcore that's ever seemed Industrial influenced to me. I can hear an Industrial influence in Powerviolence for sure but not in Grindcore.

Exactly. That's why I say 80s hardcore was clearly a subgenre of punk but later hardcore is its own thing by and large.

I..I dunno, I'll just shut up, say I'm wrong and stop thinking about this.

There's not really any right or wrong here. It's a lot of gray areas. We're just choosing different places to draw boxes around things for different reasons.

Yeah, you're right, I guess my perception of punk is just really broad.

^This

Playing fast for sake of playing fast

As opposed to what? Playing fast to power an airplane?

As opposed to, for example, learning how to play over actual chord changes or at least having a clear melody line.